Water-resistant polyvinyl alcohol filament and process for producing same



'1948, now abandoned.

-point, within six Patented Sept. 16, 1952 2,610,360 WATER-RESISTANT POLYVINYL ALCOHOL FILAMENT AND PROCESS ING SAME,

Edward T. Cline, Wilmington,

FOR PRODUGJ I per, Paul s. Pinkney, Niagara Falls, N. Y., and Louis Plambeck, Jr., and Halsey B. Stevenson, Wilmington, Del., assignors to E. I. du Pont deNemours & Com pany, Wilmington, Del., a' corporation of Delaware No Drawing. Application April 22, 1950, Serial No. 157,634

Claims.

' This invention relates to synthetic fibers. More particularly, it relates to novel fibers from polyvinyl alcohol.

. This application our application 'Ser ial'No. 19,444, filed Aprils,

I It is known that'polyvinyl alcohol can be spun into fibers which, when oriented by drawing, possess high tensile strength. In addition, these fibers have the advantages of insolubility in organic solvents, e. .g. dry, cleaning solvents, and of a highsoftening point which permits ironing when dry. However, theyhave the great disadvantage of water-sensitivity. Ordinary polyvinyl alcohol yarn dissolves rapidly in .water, particularly at elevated temperature. This has counter acted' the advantages mentioned above to the extent that polyvinyl alcohol fibers have found no practical utility in the textile industry. Methods have been proposedwhereby the water sensitivity of the fiber can be eliminated or considerably reduced by chemical treatment with an insolubilizing,agent. However, such treatments are expensive and sometimes introduce undesirable physical characteristics in the textile made from the treated fibers. There was therefore a need for polyvinyl alcohol fibers possessing resistance to water andin particular, resistance to shrinkage therefrom, even at the boiling point, and free from the objectionable aspects of. added insolubilizing agents.

This invention has as an object a highly oriented, high tenacity polyvinyl alcohol. fiber which, without chemical after-treatment, is resistant to boiling water. A further object is a process for preparing such fibers. Other Objects will appear hereinafter.

For the sake of accuracy, a water-resistant polyvinyl alcohol fiber is here defined as one which shrinks less than of its length when immersed under substantially notension in water at 100 C. during a six-minute test. No-previous sample of polyvinyl alcohol has heretofore been known to give a fiber capable'of withstanding *such a test without chemical after-treatment. I,

is a continuation-in-part of Fibers from ordinary polyvinyl alcohol are com- 'pletely dissolved by water, even below the boiling minutes.

The resistance to water of polyvinyl alcohol fibers may also be tested in another way, involving fdry tenacity, or tensile strength, upon immersion in water for ten minutes. This temperature the temperature atwhich they lose 10% of their I 2 s tion it is above 95 C. In the great majority of cases, the fibers of this invention shrink-less than 6 when immersed in water at 100 C. for six minutes, and their 10% tenacity loss temperature is at least 100 C.-and usually at least 102 C.

This therefore represents a preferred modification of the invention. I p

The above-mentioned objects are accomplished by the process of the present invention wherein vinyl acetate is polymerized by bringing it incontact-with a polymerization catalyst in a' liquid medium which is a solvent for the monomerand the polymer, the polymerization is interrupted (a) Before the concentration of the polymer-in the polymerization medium exceeds thereof,

(b) Before the conversion of monomer to polymer exceeds %,,and I j (c) Before the viscosity of the'polymer solution exceeds about 75 poises,

the polyvinyl acetate so formed is hydrolyzed'substantially completely to polyvinyl alcohol, i. e.,

to polyvinyl alcohol of saponification numberjnot exceeding 10, the polyvinyl alcohol so formed is 'spun from aqueous solution to form a filament.

the filament is oriented by stretching to a length at least threetimes as great as that ofthe spun filament, washed and dried, the dried, oriented filament further oriented by stretching, at a temperature between 200? and 240 C. to a final length at least two and one-half times the previous oriented length andthe thus oriented filament is relaxed by heating the same at a temperature between 210 and 255 C. but not more than 10 C. below the temperature of the second orienting,

under tension controlled to prevent shrinkage of more than 35%, and until the'filaments shrink not more than 10% when immersed for six minutes in water at C. but in anycase for not over thirty seconds. While shrinkage is not necessary, itis preferred that the conditions of tension be controlled to permit some shrinkage to take place. 'Thisstep in practice-is effected by running the filament from a feed roll to take-up roll and heating the filament between the .rolls, with the take-up roll rotating at a speed rangin from the same speed as that of the feed roll to a speed 35% slower than that of. the feed roll. At the first-mentioned speed ratio essentially no shrinkage will take place, and at the secondmentioned speed ratio shrinkage will take place up to 35% of its length. In all instances the filament will be under some tension because of its inherent tendency to shrink (relax) when heated.

concentration of 0.1%.

-ceed 10.

The tension is greater when the rolls are at the same peripheral speed and decreases as the peripheral speed of the take-up roll decreases relative to that of the feed roll.

The principles underlying the extraordinary re sistance to Water of the fibers obtained by the' present process are not clearly understood. There and which is a polyvinyl acetate of highlinearity,v characterized by a minimum of chain H l. e., branching, is apparently necessary. This is indicated by the fact that when a suitable polyvinyl acetate having the characteristics vjust mentioned is hydrolyzed, the ratio of its degree of polymerization to that of the resulting polyvinyl alcohol is much lower than usual for commercial materials, approachingunity in the most favorable cases, and the. ratio of the molecular weights approaches the theoretical values of 1.95.

Accordingly, the ratio of the intrinsic viscosity of the polyvinyl alcohol to that of the polyvinyl acetate ismuch higher than usual for commer- 4 mentioned can be prepared from polyvinyl acetate made at very high polymer conversion, but

'this hpolyvinyl alcohol does .not yieId Waterresistant fibers when subjected to the remaining steps of the present process.

1 In connection With the third and fourth steps, the particulardetails of which will be discussed "more fully hereinafter, it is to be observed that both of these specific steps must be used with the particular polyviny1 alcohol described above, since the polyvinyl alcohol ordinarily produced fails to'produce' water-resistant fibers even when cial materials. To obtain a polyvinyl acetate that will yield a polyvinyl alcohol which by the subsequent stepscan be converted into the water resistant fibers, it is necessary, as pointed out in connection with1the second step, that the polymerization of the vinyl acetate be interrupted at. thestated viscosity, polymer conversion, and polymer concentration. In general, the polyvinyl acetate thus obtained will have a relative viscosity in chloroform at C.', withinv the range of 1.03 to 1.18 and preferably 1.07 to 1.18, at a The remarkable differences in behavior among individual vinyl esters, even those which are chemically very closely related, are shown by the fact that other vinyl esters e. g., vinyl-.propionate or vinyl benzoate, do not give a polyvinyl alcohol suitable for the preparation of water-resistant fibers. ,Vinyl formate, on theother hand, is also capable of leading. to water resistant polyvinyl alcohol fibers, as described in application ElenNo. 157,632, filed of even date by Hatchard and Hill. Even vinylformate, however, is unexpectedly different from, vinyl acetate [in that its polymerization need not be carried out in, amedium which is a solvent for the polymer, and in that it yields water resistant polyvinyl alcohol fibers (as definedalcove) even when the last step of the process, i. e., the heat relaxation step, is omitted.

In the second step, the polyviny1 acetate is at least 99%. hydrolyzed, i. e.,hydrolyzed to the extent-that the saponification number of the resulting water-soluble polyvinyl alcohol does not ex- An important factor is the molecular weight of the polyvinyl alcohol, which apparently should notbe too high. To remove the uncertainties attending molecularweight determinations ofhigh polymers, it is preferable toadopt as a'criterion the relative viscosity in a suitable solvent. It has been found that the polyvinyl alcohols suitable for the production of waterresistant fibers have a relative viscosity in the range of 1.1 to 1.3 in 85% phenol at 25 C. and 0.1% concentration. However, polyvinyl alcohol of this viscosity range is not in itself sufficient but must be coupled with the previously mentioned requirement of interrupting the polymerization subjected to these spinning steps. Even the highly linear polyvinyl alcohol prepared under the conditions disclosed herein from polyvinyl acetate will not yield water-resistant fibers when spunby methods not involving both the thermal stretching and final heat treatment required by the present process.

The following is a brief description of a method for carrying out the process in a preferred embodiment. Vinyl acetate is polymerized'in an organic solvent in which the polymerissoluble, e. g. methanol, in the presence offanazonitrile catalyst, or initiator, such asl alpha,alpha'.-azo,-

bis(alpha,gamma dimethylvaleronitrile) The polymerization is preferably. carried outatrelatively low temperature, e; g. between 0? C. and 50 C., and it may be accelerated by the use of ultraviolet radiations. The, polymerization is stopped before the concentration of the polymer in the polymerization solution. reaches about 50%. Theseconditions, as have been noted previously, are believed to minimizechain-branching and favor high linearity. The polyvinyl acetate is then converted to polyvinyl alcoholbytreatfment with methanol in the presence of catalytic amounts of sodium ,methylate. The polyvinyl alcoholis dissolved in waterandE'Lthe aqueous solution isextruded through a. spinneret' into a' salt coagulating bath, preferably a saturated solutionof monosodium phosphate. The-filaments are given a preliminary stretch as a desirable step preliminary to washing and, drying. After washing with cold water to remove the residual salts and drying, the filament'sare highly oriented by hot stretching in a zonemaintained at a temperature of, 2l0-235 C., the draw ratio being preferably above 3/1. The oriented filaments are thenallowed to relax under controlled tension permitting shrinkage up'to 35% of, their length in a zone at a, temperature the same as or somewhat higher than the stretching temperature, e. g. 225-2e0 C. The filaments so ob tained will, without any chemical after-treatment, shrink less than 10% when treated with water at C. for six minutes. .In the most favorable cases, they even'resisatheaction of water under pressure at temperatures ashigh as C. for ten minutes. V

The invention is illustrated in further detail by the following examples in whichthe parts are by weight.

' Erample I aiidthesol-ution 'was stirred underan atmqs phere of nitrogen at reflux temperature for two 1 hours. Atthe end of this time, the polymerization was terminated by adding approximately 0.2 part of m-dinitrobenzene as inhibitor. Thesolution was; steamedto remove benzene and unreacted vinyl acetate; andy-the residual polyvinylacetate was dried by milling at 140-150" C. The yield was 418 parts (35% conversion) of polyvinyl acetate. The concentration of the polymer in the polymerization solution when polymerization was arr'estedwas 14%. It had a relative viscosity in chloroform at 25. C. and 0.1% concentration of 1.091.

For conversion to polyvinyl alcohol, the polyvinyl acetate was dissolved at concentration in dry methanol. --The=s'olution was heated to reflux temperature and treated, at 5-minute intervals, with four portions each of 0.5 part of sodium. methylate dissolved in eight, parts of methanol.- The alcoholysiswas then allowed to continue for one-half hour, after which the polyvinyl alcohol; which had separated in quantita-.

tive'yield, was filtered off,washed well with methanol, and oven-dried-at-70j C. It had a saponi fication number of 5 and a relative viscosity in C. and 0.1 concentration of" 8.5% phenol at 25 1.132. Thus, the ratio of the intrinsic viscosity of *thepplyvinyl alcohol to that of the polyvinyl acetatewas 1.42 (intrinsic viscosity has its usual meaning; as defined, for example, in Advances iniColloid-science, vol. II, age 209 or;;in U. S. Patent 2,130,948, i. e.. it is the logarithm of the relative viscosity divided by the concentration in grams per 100 cc). This ratio, which is much higher thanthe usual ratio of about 1 or less for commercial production polyvinyl acetate and polyvinyl alcohol, indicates high linearity in the polymer.;

aqueous solution of this polyvinyl alcohol was prepared bydissolving it in hot water containing along chain, ester as a stabilizing agent, this material being present in an amount of 0.14% based on the total solution. The solution had a viscosity of about 16 poisesQItwas spun Ithrougha spinneret navmg eo holes M0004" diameter. into a coagulating bath at 26 solution of monosodium phosphate containing 0.02% of a surface-active and anti-sticking agent containing of octadecyltrimethylammonium bromide as the activeiin'gredient. After60 inchesv o'f bath travel involving passage around two yarn driven rollersfthe yarn was. wound around a Godetwheel' runningiat a peripheral speedof From the Go'det wheel the 377 .per. minute.

dispersible phosphate C.'consisting of a aqueous yarn: was passed through air to a windup bobbin run'ningfat:1385" stretch at a draw ratio of 3.67 /1. This amount of stretch is desirable to prevent sticking during the subsequent washing with water and drying.

1The-; yarn was 'washed thoroughly ,on the,

bobbin with cold water 1.1)0'16II1OV8 the salt deposited on it from'the' coagulatingbath, airdried ,at room temperature, twisted 1.21 turns per inch, then oven-dried at 'C. The yarn was then orientedby stretching it in air through a .tube 32"- long, heated to 220 Cxby means of a jacket containing the vapors of aj=boiling liquid-heat transfer agent, which in this: casewas an eutectic mixture of; diphenyl and diphenyl ether. The input. speed was 50 feet per minute and the draw ratio was 3.2/1. After stretching, the yarn was continuously relaxed by permitting it lengthli. e.,.to' 82.5%-

per minute, thereby imparting 0.1 1 en p e ise:-

to shrink 17.5% of its 6 ing) by running it through the same tube main-i tained at'a temperature of 235 C.'at.an input speed o-175 feet ,-per minute, shrinking being permitted? and controlled by running the take up roll at a 17.5% slower peripheral speed than the feed roll.,

The polyvinyl alcohol yarn so obtained hada denier-of 43, adry tenacity of 7.0 g./d. with 14% elongation anda loop tenacity of 6.6 g./d. with 13% elongation. Its sticking temperature was 217 Theyarn shrank only 7% when boiled inwater at C. for six minutes. This test wascarried out by tying, a small weight,

less than; 1 mg. per denier, on the yarn to keep it from floating when lifted by the steam bubbles, and immersing a measured length of about 10 centimeters in water boiling vigorously at'100 C. After six minutes in boiling water the samplev was removed, dried and measured again. Experience has shown that practically all; the shrinkage occurs during the first six minutes of. immersion, and in fact during the first few seconds, for fine denier yarn. Boiling for one hour "instead-of six minutes usually'does not increase the shrinkage by more than 1% overthat; given'by the six-minute treatment.

X-ray difiractionpatterns of the relaxed yarn showed j it to be highly oriented and very highly crystalline. .It appears that the combination of 7 high orientation with high erystallinity is necessary for water-resistance since yarns with, either property alone in a high degreeare not resistant to boiling water. v

Example II A solution of 2400 parts of vinyl acetate (distilled from copper resinate-inhibited stock) in 600 parts of dry methanol containing 12 parts (0.17 mol percent based on the vinyl acetate) of alpha,alpha-azobis(alpha,gamma-dimethyl-- was placed in a glass vesselprovidedwith stirrer,

thermometer, nitrogen inlet tube and take-01f tubefrorn the bottom forsampling. The vessel was placedin a water bath maintained at 18 -20 C. -and two 275 watt lamps 'emitting-lightrich -in. ultra-violet radiation were placed as close as possible to the flask. I Irradiation was started and the solution was stirred under an atmosphere.

ofnitrog'en. Duringthefirst hour of polymerization the temperature of the solution varied between 20. and 25 C. It was maintained at 25 C.;after the first .hour. Weighed samples were removed at intervals'to follow'the course of the polymerization. After 3% .hours, asarnple- (1426 parts). was removed and the polymer was isolated by adding a trace of m-dinitrobenzene as inhibitor, steaming out the methanol and unreacted monomerand milling dry at --150 C. There was obtained 300 parts of polyvinyl acetate, indicating a conversion to polymer after 31/ hours of 26.3%. The concentrationof the polymer in the solution at the end was 21%. The polymer had a relative viscosityin chloroform at 25 (land 0.1% concentration of 1.104, and a number average molecular weight, byosmotic pressure measurement, of 110,000.

' ThisIpolymer was quantitatively converted to polyvinyl alcohol by the following ,methanolysis procedure:

A 14.3 solution of 200 parts of Y some? a e ate em ih eo a a ded av r-.-

an hour period to 1200 partsof refluxing m'et'h-' anol containing parts of a 30% solution of sodium methylate in methanol. Half way through the addition a second 5 parts of the catalyst was added, and a third portion at the end. The resulting polyvinyl alcohol was filtered, washed thoroughly with methanol and oven dried at 85 C. for 16 hours. Thepolymer had a saponification number of 8, a relative viscosity in 85% phenol at 25C., and 0.1% concentration of 1.172, and a number average molecular weight (by osmotic pressure measurement) of43,800. The ratio of the intrinsic viscosity of the polyvinyl alcohol to the intrinsic viscosity of the polyvinyl acetate was 1.61, and the ratio of the degree of polymerization of the polyvinyl acetate to that of the polyvinyl alcohol was 1.29. These figures indicate high polymer linearity.

The polyvinyl alcohol was spun from' a 13% aqueous solution containing 0.14% of a phosphate ester stabilizing agent'as described in Example 1 except that the bath travel was 120", involving passage around four yarn-driven rollers. The yarn was then given a preliminary cold stretch at a ratio of wind-up speed to Gjodet wheel of' 3.12/1, washed and dried as in Example I, then hot stretched in air at 225 C. at'a draw ratio of 3.2/1. The oriented yarn was then relaxed continuously by shrinking it 20% (to 80% ofits ori-- ented length) at a temperature of 232" C. 7

The polyvinyl alcohol yarn so obtainedhad a denier of 48 and a dry tenacity of 8.0 g./d. with 16% elongation. It shrank only 6% when-boiled in water at 100 C. for six minutes. After boiling in water for" one hour, the yarn was still uniformly strong and had'a drytenacity of 7.0 g./d. with 20% elongation. Itswet tenacity in boiling water was 3.3 g./d.. While the yarn was practically completely unaffected by boiling water and even by water at 105 C., it could be dissolved by water at 110 C. or by boiling 35% aqueous pyridine. V v

When yarn spun and hot drawn in the same manner from the same polyvinyl alcohol was relaxedby shrinking it 2% (to 98% of its oriented length) at 235 C.,it shrank 4% inwater-atl00 C. during six minutes; had a denier of 39 and a dry tenacity of 10.5 g./d. with 6% elongation'. When another sample of the same, yarn was relaxed by shrinking it 29% (to 71% of its lengthy at 235 C., itshrank 1% during six'minutes exposureto waterat 100 C., hadfa denier 01-53, and a dry tenacity of 7.0 g./d. with 29% elongation.

Even when no shrinking was permitted (1; e., the yarn after stretching was recirculated through the cell at 235-C.'with feed roll and take-up roll at the same speed) the yarn was resistant to water. If this second heat treatment is omitted,

however, water resistance is not achieved, 1. e.,

the stretched yarn itself is highly sensitive to boiling water. V H

When the relaxingstep was carried out' at 8 tion, dissolved completelyin water at91 C.*w'ith-' in'six minutes. nection with the above experiments, that yarns made from polyvinyl alcohol of high linearity, as described herein, can' be drawn to appreciably higher draw ratios than yarns made from the usual types of polyvinyl alcohol.

Example III To a-solution of 500 parts of vinyl acetate in 125 partsof methanol-at 40 C. was added 1.84 parts of -alpha,alpha-azobis(alpha,gamma-dimethyl-gamma-methoxyvaleronitrile) Polymerization was carried outfor ZAlhours at 4042 *C.,' after which it was stopped by addition of m-dinitrobenzene. The polymer was isolated asin Example II. The yield was .208parts of polyvinyl acetate, corresponding to a conversion of 41.6% and to a polymer concentration at the end of 33%. The polymer had a relative viscosity in chloroform at 25 -C.'and 0.1% concentration of 1.127. I

This polymer was hydrolyzed under the-c0nditions described iri- Exa'mple II. The resulting' polyvinyl alcohol had a saponification number of 7' and a relative viscosity in 85%- phen'ol at 25 C.- and 0.1% concentration of 1.192. The ratioofthe intrinsicviscosity of the polyvinyl alcoholto' that of the polyvinyl acetate wasIAl.

The polyvinyl alcohol was made int'o'a 1 2.5% solution in water, the solution containing also 0.14% of a long chain, dispersible phosphate ester. The solution had a viscosity of 25fpois'es. It was extruded into a coagulating-bath at 25 0., all spinning-washing and dryingconditions being as described Example I. V The dry yarn was stretched at 225- C. and at a draw'ratio of 3.6/1,

' then relaxed by permitting it to shrink 20% "at 220 C., i. e., 5 C. lower than the stretching temperature, it was still possible to relax'the yarn although the maximum shrinking was 7". The

yarn shrank 4% in water at 100 C. after sixminutes exposure and wasstrong after boiling.

For purpose of comparison, a medium viscosity, highly saponified polyvinylalcohol from'standard commercial production was spun from a 14% aqueous solution as-described in Example I, given a-preliminary cold stretch at a draw ratio of 3.4/ 1, washed, dried, hot drawn 2.8/1 at 220 C. and relaxed 20% at 220 C. The resultingyarn, which had a dry tenacity of 7.2 g./d. with 17% elonga- 233 C. It shrank only 4% when subjected to" the action of boiling water for six minutes; It had a dry tenacity of. 7.1 g./d,"with 16% elon gation. V I

-A polyvinyl alcohol of. higher relative viscositythan those of the: preceding examples-was pre pared as follows. To a solution of 1100 parts of vinyl acetate in 275 parts ofmethanol at 30 C. was added 4.05 parts of;alpha,alphaf azobis(alpha,gamma dimethyl --gamma methoxyvaleronitrile). Polymerization was carried out at- 30-31 C. for 4% hours; at which time: thersolu tion was highly viscous; Polymerization was stopped by'addition of m-dinitrobenzeneand the polymer was isolated as. previously described. The yield wasi345 parts, correspondingto a 31.4 conversion and a polymerconcentration at the end of 25%.. Theresulting polyvinyl acetate; which had a relative viscosity'in chloroform at 25 C. and 0.1% concentration of 1.142, was hy drolyzed' as in ExampleII. The pjolyvlnyl'alc'ce hol had a saponification number of'7'and'a rela tive viscosity in %--phenol at" 25 C. and-01% concentration of 1.235. The ratio ofthe intrinsic viscosity of the polyvinyl alcohol to that or the polyvinyl acetate was1.59;

The polymer was'made into'a -10.5% aqueofis-' solution which had a viscosity of '27 poises. This solution was wet spun,f all conditions; ofspin- It should be observed, in con-- V phate at 29 I.? *The yarn was drawn at 235;C. at a-draw ratio of 3.6/1 and relaxed by permitting it to 'shrink' 22% (to .78% of its'drawn lengthyat 235 C. The relaxed yarn had a dry tenacity of 5.4 g./d. with 16% elongation. It shrank only 6% of itsylength whensubjected to water at 100 C. for six minutes.

Example V A stainless steel kettle was charged with 17,217 parts of vinyl acetate and 1'722 parts of iso propylalcohol. The kettle was heated .to 50 C. and'a solution of 24.5 parts o'f alpha,alphaazobis(alpha,gamma-dimethylvaleronitrile) in in methanol was addedfto terminate the reac-f tion. ..Withdrawal of asample andjworking up ofgthe' polymer therein showed. 31% conversion tolpolyvinyl. acetate. Therelative viscosity of the polyvinyl acetate was'1.0 85 in 0.1% concentrationfiin chloroform at 25 C. The reaction product in the. kettle was concentratedunder reduced pressure, replacing the unchanged monomer and, solvent with methanol." There was finally obtained a methanol solution containing 15.2%,"of polyvinyl acetate. v

i For the hydrolysis step, a stainless steel kettle was charged with 13,600 parts of methanolfand 200 parts of a solution of sodium methylate in methanol. This was heated to C. and the polyvinyl acetate solution was added at the rate of about 450 parts per minute with stirring. After one half of it had been added, 200 parts more of the sodium methylate solution was run in and an additional 100 parts after all the polyvinyl acetate solution had been added. The total operation lasted one hour and 23 minutes at 55 C., after which the mixture was stirred for an additional hour at 55 C. The polyvinyl alcohol slurry was filtered and the solid hydrolysis' product was stirred with 15,000 parts of methanol and 100 parts of acetic acid, filtered again and the same operationv repeated twice more using only methanol. trifuged and dried at 50 C. underreduced pressure. Its'relative viscosity was 1.144 at 0.1% concentration. in 85% phenolat 25 C. The ratio of the intrinsic viscosity of the polyvinyl alcohol to t t-mime p l v nv aceia ewas -6 {1 polyvinyl alcohol had a saponification number of about 3.0. I q y A 17% aqueous solution of the polymer in water containing 0.14%, based on the total solution,

' of a long-chain, dispersible phosphate estezffas aastabilizing agent was extruded through I a.

spinneret having holes of 4 mil diameter into a 45% aqueous solution of mono'so'dium phos- C. Thecoagulating set-up included two yarn-driven rollers, which gave a very slight tension to the filament. The bath travel was 60 inches. After removal from the bath, the.

wet filament was stretched in air at a draw ratio, of 3.7/1, wound up on a bobbin, washed thoroughly on the bobbin with water, dried on the bobbin and finally it was given a twist of 1.2 turns per inch.

This yarn was drawn at 220 C. to a ratioof 3.8/1, then permitted to shrink 13.5% at 230 C. It was then water insensitive as shown propyl alcohol was added. The polymerization The final product was .cen-.

mersion in boilin'g water for six fminu'te'smnd." had'a 10% tensile. loss temperature of 105C.

In comparison, a commercial'medium viscosity polyvinyl alcohol was wet spun under similar" conditions with a .total drawratio of about 3.7/1, then drawn at 215 C. to a ratio of-.2.6/1, and finally permitted to shrink-20% at 220 C. After. this operation, the yarn was still soluble in boilingwater. Q v

1. a I 'Ewample VI n I "catalyst suitable for. polymerizing vinyl acetate was prepared by shaking a cooled; solu-: tion of 2.5 parts of sodium perborate in 100parts of-water withan equal volume of cooled vinyl acetate for about three minutes, separating the vinyl acetate and drying it over sodium sulfate. Vinyl acetate so treated contains diacetylerq id Y 1 A'mixture of 800 parts of vinyl acetate and 250 parts of beta-methoxyethanol was heated to 50 C. with stirring under a nitrogen atmos- I ingjapproximately 0.2 m ole'per cent of peroxidef based on the total vinyl acetate present. After an induction period of about 25 minutes, poly-Q merization startedand was allowed to proceed for 1 ..hou rs to a conversion of 26%. The poly?" mer was isolated --and hydrolyzed as in Exam.- ple I. The .relative viscosity of the polyvinyl. acetate in 0.1% solution in chloroform was 1.108; that of the polyvinyl alcohol in 0.1% solution in phenol 'was 1.154, and its saponification number was zero. The ratio of the intrinsic viscosity of the polyvinyl alcohol to that of the polyvinyl acetate was 1.40. j

Anaqueous' solution containing 16.5% of this" polyvinyl alcohol and 0.14% of a long-chain dispersible, phosphate acid ester was prepared and wetspun as in Example I. The washed and dried yarn was drawn at 220 C. to a ratio of 3.8/1 and permitted to shrink 13.5% at 230 C. It shrank 5% in waterat 100 C. and had'a 10%.tenacity loss temperature of 102. "Its dry" tensilerstrength was 7.7 g./d. at 11%"'elongation.:

I process as follows: .a reaction mixture was pre pared consisting of 3300 parts of vinyl-acetatef 400 parts of "isopropylalcohol' and 700 parts'of vinyl acetate containing diacetyl peroxide, prepared as in Example VIII, The charge contained approximately 0.2 mole percent of peroxide based on the total vinyl acetate. This mixture was forced at a constant, predetermined rate into a pressure vessel provided with anoverflow pipe and maintained at 50 C. and the partly polymerized reaction mixture was collected con tinuously at the overflow. The conversion was approximately 24% after the polymerization reaction had reached a. steady state. The polymer had an average relative viscosity (in 0.1% chloroform solution at 25C.) of 1.092. I

Hydrolysis was carried-out in an aqueous acidic. medium as follows: a mixture of 210 parts of the above polyvinyl acetate, 700 parts of water, '7 parts of 95% sulfuric acid and 4.2 parts of a sulfonate-type dispersing agent (an alkyl-arylv sulfonate of. active ingredient) was heatedwith stirring at 05 C- for six hours. The resulting homogeneous solution was then .steamdistilled' until. the'distillate was free from acetic acid. Th polyvinylals ohol. is l t dfroma pgrtioaot.

l1 therzsolution ":had arelative viscosityflin 50.1% 85%lphe1iol solutionat.-25.C.) ct-1.1481 .z'lhe' ratio: or the intrinsic viscosity of the polyvinyl alcohol'to that of the polyvinyl acetate was 1.57.

.The aqueous hydrolyza'te 'was neutralized to a pH' of 6.4 by addition of sodium hydroxide and concentrateddown to. spinning viscosity. The concentrated "solution contained about 12.5%:of polyvinyl alcohol. It was wet spun directly by the procedure of Example 1 except that the Godet wheel and wind-up speeds were 302 and 1103 inches per -minute, respectively. After washing and-"drying, the yarn was drawn at 225? C. toa ratio of 3.6/1, then permitted to shrink 13.5 %'--at 225- C. It shrank 3%inwater at 100 C: and had a tensileloss temperature of 105 C.

Thepolymerization catalysts used in the first step for-polymerizing the monomeric'ylnyl acetate are preferably the free radical-producing catalysts or initiators. Among such agents "may; be mentioned the peroxy catalystsuchas the a'cyl-percxides, e. g., benzoyl peroxide, *acetyl peroxide, lauroyl peroxide, etc. and the alkyl peroxides such as diethyl peroxide or t-butylhydroperoxide. Other suitable'agents are photoacti vators like benzoin or diacetyl in conjunction with ultraviolet light. A particularly useful class of free radical-producingcatalysts are theazo compounds disclosed in Hunt U. S. Patent 2,971,959, i. e.,'thecompounds having anacyclic azo group, -N=N-'; bonded to. different carbons which are non-aromatic in character'and of which" at least "one .is tertiary'and in which the tertiary carbon'has attached to it through carbon a radical in which the three remaining valences'of the latter carbon are satisfied hyat least one element of atomic number 7 or 8 (oxygen and/or nitrogen). By far, the most useful class of catalysts, becauseof their outstanding activity at low temperatures, are the symmetricalazonitriles disclossed in U. S.Patent 2,471,959 already referred to and in application Ser. No. 2,552, filed by J. A. Robertson on January 15,. 1948, now U. S. Patent No. 2,586,995. "This class of azoriitrile catalysts comprises the organic compounds'having anacycl'ic azo group, N=N--, bonded to two different carbons'whic'h are nonaromatic, i. e., aliphatic or cycloaliphatic, each 01 said carbons being tertiary, i. e., attached to three other carbons by single valences, and hav-, ing .a v cyan'o "group, CN, attached thereto. Thesecompoundsthus have the general formula men N- R.

whereinjthe Rs aremonovalent organic radicals, ortwo of the Rs attached to the same carbon form together a'fiveor six membered alicyclic ring. Preferably, the Rs are hydrocarbon radi- 6O cals orxhydrocarbon radicals submitted, With alkoxy groups of one to four carbons. The most effective of these symmetrical azonitrile catalysts are those in which the radicalsattached to the 12' isobutyli g'amma-methylvaleronitrlle) ,1 .1:al pha,alpha' azobis alpha-,gamma 'dimethy'l Qga'mmamethoxyvaleronitrile) 1,1 azodic'ycloh'exane-' Chem; '-"Weekblad, 23, 7'7 (1926) and DoxpJ. Amer. Chem." Soc. 4'7, 1471' (1925') -o'r'-*'by the 7 method described in U. S; Patent 2,469,358.

The amount of catalyst, based o'n the-weight of th-e'vinylfacetate', may boas low as 0.001 %*or-as high as' 3'%- or even more. Y range between 0.05% and 1%;

'P'olymerization'of' the vinyl acetate may be carried out either by the'bulk'methodor in solution.

Ineither case, of course; thepolymer as itforms is actually in solution; the solvent being either the "unchanged monomer or an extraneous organic solvent. It is easier to control the course of the polymerization when an additional 'solvent is used and thismethod is therefore preferred. The choice'of a solvent ffQT'the 'pol-ymerl zationis not critical." It is only necessary "that the solvent be'one'which dissolves the polymer, eat to the extent of at least 5%,,a's well as the monomer; that it. be .unpolymerizame under the reaction conditions, and f of course that "it be essentially chemically inert toward the vinyl aceta'te and the catalyst There maybe used hydrocarbons 1 such as toluene, xylene, cycloh'exane;

alcohols such as, ethanol, isopropyl alcohol or butanol; ether-alcohols 's'uch'as methoxyethanol; ketones su'ch'as acetone; esters. such as methyl acetate, ethyl acetate or ,butyl acetate; heterocyclics such as tetrahydrofuram etc. The pre-' ferred solvents are the alkanols .of .1 to lc'arbon atoms and particularly the secondaryalkanols of 1 toil carbon 'atorns such as isopropyl alcohol and secondary butyl alcohoL; since these solvents lead to polyvinyl acetate within the optimum range of molecular weights. Mixed solvents may also be used. For example, a 5:1 mixtureby weight j of; methanol and isopropyl, alcohol is a as long as the polymer remains in solution. ."--A.' range of proportions suitable for most purposes is between-5% and-300% of solvent based on the weight, of monomer. *With solvents Whichact-as molecular weight regulatorssuchas the preferred low alkahols; it only *necessary to use ---small amounts, e. g. "between and 20 "by weight of the monomer, although desired.

' The polymerization"temperature is desirably kept relatively low; e. -g., below-100 (3., and-preferably below C. With' active catalysts like the'tazonitriles, it is possible topolymerize'v'myl acetate at temperaturesaslow as-0- C. or even less. The preferred range of polymerization'tem perature with these catalysts is between '25 and 50 C. With peroxide-:type catalysts,-- the *pre-" ferred range of polymerization temperature is between 50 and 80 C.

The polymerization should be interrupted be-' fore gelationoccurs, 1. e.;-beforethe polymeriza- A generallypreferred more can be used if .tionsolution ceases tobe fluid. This is to prevent undesirable chain branching and production of polymers from which water resistant fibers cannot be obtained. It will be'understood that it is not possible to specify accurately for all polymerizing solution (regardless of whether the solvent is solely unchanged monomer or whether an additional solvent is present) exceeds about 60%. It is often desirable to keep the polymer concentration below about 45%. inv order 7 to achieve the best results. Another criterion which may be used advantageously from the practical standpoint relates to the viscosity of the polymer solution. It has been found that the polymerization should be interrupted before the polymer solution (in unchanged monomer or in added solvent) has a viscosity exceeding about '75 poises. Here again no absolute limit can be given and small variations may be tolerated in particular systems. While the polymerization can be interrupted at any desired point before the upper limit of polymer concentration is reached, it is of course uneconomicalto do so too early. Thus, in general, the polymer concentration will be at least 1% and preferably at least It may be noted that while polymer yield, 1. e.. the per-- centage of monomer converted to polymer, could be used as a test of the desired extent of polymerization, it is notentirely satisfactory for the reason that the possibility of gelation is affected alsoby the molecular weight of the polymer and the concentration of the solution. thoughv it is in general found that the polymer yield does not exceed about 65%, in some cases it is possible, tohave conversions of 75% or even higher without reaching the point of gelation. The polymerization may be stopped by any conventional procedure such as precipitating the polymer by addition of a non-solventorby adding one of the" known polymerization inhibitors such as m-dinitrobenzene, thiourea, etc. Unreacted vinyl acetate can of coursebe recovered and used again. v r

The hydrolysis of the polyvinyl acetate to polyvinyl alcohol is preferably carried out either by alcoholysis, i, e., ester interchange, or by treatment with water containing an acidic catalyst. The alcoholysis may be carried out withv any desired alcohol but it is preferable to use an .alkanol of 1 to 4 carbon atoms, e. g. methanol, ethanol,

propanol, isopropanol, n-butanol or the isomeric Thus, al-

butanols, or an ether-alcohol such as methoxyethanol, which gives very good results. The preferred alcohol is methanol. The catalyst is preferably' an alkali-metal alkoxide, which for'convenience is'preferably the alkoxideof the alcohol used inthe alcoholysis process, and it is suitably used in amounts between 0.05 and 5%, based on the weight of the polyvinyl acetate.

The alccholysis proceeds at any temperature but, for a practical reaction rate,- a temperature above about 20 C. is desirable, .the preferred range being between 50 and 120 C. The conversion is quantitative. The product separates from its alcohol solution as the hydrolysis proceeds and it can be washed with suitable non-solvents such as alcohols to remove the catalyst which may. be

not exceed 10. Under "the condition described,

the polyvinyl alcohols will have relatively ,1ow

molecularweights, as judged from their viscosity in solution. The polyvinylalcohols giving waterceeding 1.3. Preferably the relative viscosity. is in the range between 1.1 and 1.3.

Hydrolysis in aqueous media is preferably carried'out with sulfuric acid as the catalyst, al-

though other's'trong acids of dissociation constant above 1x10 e. g. phosphoric acid, maybe used. The hydrolysis temperature isprefer'abl'y between '75 and'100 C., and it is desirable'to f use a wetting agent, preferably a sulfonate or. sul fate-type wetting agent. This methodTha's. the

great technicalfad'vantages that, after completion of the hydrolysis and steam-stripping of the acetic acid liberated followed by'neutraliz'ation with an alkali metal hydroxide, the polyvinyl alcoholsoliimay be Spun i ectly without isolating the V polymer.

The polyvinyl alcohol isspun from aqueous solutions either by the dry spinning orthe we't spinning process. While dry spinning has-certain advantages, such as permitting a high rate of extrusion, it involves certain complications not present in'wet spinning, which is therefore" preferred. Wet, spinning of thepolyvinyl alcohol, is carried out from aqueous. solutions in which the solvent may be Water alone but is preferably water containing a stabilizing agent, i. e., an agent which prevents or decreases the formationof gel particles on standing. There may beused, for instance, between 0.05 and 0.5% 'of a phosphate ester as shown in the examples, or larger amounts, e. g. about 5% to 15% of a water-soluble amine such as pyridine or Water-soluble aliphatic alcohol such as ethanol. Wetting and softening agents may also be included. Preferably, the solvent should comprise at least water. The concentration of the polyvinyl alcohol in the spinning solution depends to some extent on its molecular weight. It can be as high as desired, provided the solution is fluid enough to handle. Concentrations between 5 and 30% are usually employed, an optimum range being 10-20%. The I about C; or higher if dry spinning is used.

The coagulating bath is an aqueous solution of one or more inorganic alkali metal saltssuch as sodium sulfate, disodium hydrogen phosphate, zinc sulfate, potassium sulfate, ammonium chlo ride-ammonium; sulfate, etc. Particularly desirable in view'ofits better coagulating properties is a bath consisting of monosodium dihydrogen phosphate (NaH2P'O4). The salt bath, should have a. high concentration in order to remove water rapidly fromthe aqueous polyvinyl alco hol solution. Preferably the saltconcentration is just below the saturation; point. Cationic surface-active agents such as cetlypyridinium bro;- mide or cetyltrimethylammonium bromide in amounts of from 0.005 to 0.05% based on. the weight of the bath are advantageous in that they promote better spinneret performance and decrease sticking orbreaking of filaments. The bath is maintained at any desired temperature between about 5 and about 100 0., preferably between 20 and 80 C.

The coagulated filaments contain salt fromthe coagulating bath, which salt must be removed before a further processing. The filaments may a P9 fles ed ith a W ter-Pre so ventm xture "acr eeo' 'suflicient" orientation. to decrease its sensitivity toH-waterto'the point where the yarnshowslittle or. ,n,osticking during washing and drying; In practice, ;the' yarnshould be stretched at" least 200 %;of its initial 'length, i. e., ,at a draw ratio of 3/ 1. The .draw ratio can be ashigh-agdesired up" to the point where excessive filament-breaking occurs; whichis ata draw-ratio of about 4/1. Washing and drying are. carriedout at fixed,fila mania-length; which is most conveniently achieved while the yarn" is on a bobbin. The yarn is washed with water preferably at" a temperature not above-25 C. until the salts "are substantially completely removed as judged by the" pH of the efiluent; ash: determination ;r; 0ther methods. Presence of residual-salt on theyarn is detrimental toeubsequeniroperations such as twisting, drawing, etc;

Beforethe hot drawing step, the-yarnshould be; relatively" dry; thatyis; in equilibrium- With-air Wi-th'a relative humidityof less than 60%, at --=C-. The 'yarnmaybe dried-"bye variety of means; e. g. in an oven at'aboutfiO 0., or-over a moisture-"absorbing material-atiord-inary temperature and reduced pressure, or-by-passingover ahotrrod-or through a hotzone, e. g. 'a tube at 1 25"C'. Since the dried yarn absorbs'moisture rapidly; tit should-beprotectedfrom humidity if itisrnotto be used shortly after drying; Before hot drawing, ;theyarnis preferably pretwisted about one to two turns peninch: This operation is-rbyno-means essential but it appearstobe beneficial in the subsequent drawing.

Thehot drawing is carriedout most economically in air but; if desired; in' some inertatmosphere suchasnitrogen. Any-suitable means-0f uniform heating may be employed; the preferred one-being a tube, jacketed with a heat-transfer agent or electrically heated, through, which the yarn-is-run; Temperatures aremeasured by means of a thermocouple insertedin the tube. The length of thetube depends-on the rate of drawing; In general, a contact-time of one-to four seconds gives the best results; anddrawing tensions between one'and two g./d. indicate a proper combination of speed-and temperature. The drawing tenv-peraturai. e., the temperature of the zone through-whichthe yarn is passed during stretching; is maintained in a range between 200 C; and 240 C. The-optimumtemperature is readily determined from the experimental limitations that too higha temperature causes-sticking of the filaments while too low a temperature causes excessive tension resulting in broken filaments. The best results are obtained in general by stretching-eta temperature between 220 and 235C. and atthe maximum possible tension short of the point"where'exces sive-filament breaking occurs; Stretching is best carried-out by means of a pair of rollers 'I'BVOIV? ing" atdifferent peripheral speeds; ratio is desirably atleast 2.5/1 and preferably at The draw 7 16 least 3/1, the maximum permisslbl'e being in general 4.5/1 for Wet-spun fibers but up .to 10/1 for dry spun fibers: It should be noted that the drawn yarns, al.

though highly oriented as shown by X-rayhatterns, are in general still soluble in water at-lilll.

Ckat this stage. a The underlying reason is per- V haps' that the stretched yarns are highly strained and deficient in crystallinity. Water insolubility isimparted to the yarn by the-next and. last stepin the process, which isa relaxing step. Relaxation of the filament or yarn may be compared to the familiar annealing process. -Itis believed that it releases strains inthefiIament and induces a higher degree of crystallinity.

The relaxing procedure consists "in heating the filamentunder controlled tension at a temperature which is preferably the same'as or somewhat higher than the stretching temperature. The relaxingtemperature, however'5cah be as much as 10 C. lower than the-stretching-temperature. The tension on the filament is's'uch that it produces no additional drawing andpreferably permits shrinking up to a maximum of about 35%, i. e., up to the point wherethe filament can shrink to 65%oi its'drawn length. Preferably, the filament or yarn is allowed to shrink between 10% and 30 of its initial" length. This is best carried out by running the yarn or filament again through the drawing cell used in. the stretching step; ora similardevioecapable of being heated, with the take-up roll at the samepor lower peripheralspeed than-the-feed roll. The relative peripheral-speeds of the rolls are adjusted so. that either no shrinking takes place, or-preierably, shrinking up to 35% ,of the initial length; It is'only with yarn-from'poly mers having the highest degree'lof linearity' that this step ,can be carried out with no. shrinkage and still give a water-resistant yarn, The re,- laxing' temperature should be Within the range of 210 to255 C the most generally'useful range being 225-245? C. Within these limits, the relaxing temperature'may; in certain. cases, be as much as 10 C. below the stretching temperature, but p-referablyit' is the same or a few degrees higher, The time duringwhich the yarn. will be in the heated zone can range fromabout one second up to'the time, ,usually not more than 30 seconds, .at which. damage to the filament (e. g. discoloration, and sticking) would, result upon. continued heating. A generally suitable heatingtime is 2-5 seconds. 1

The relaxed yarns obtainedlbywtheuprocess just described are insolublelinwaterat 106C. Their tenacity depends to some extent on, the amount of relaxation; For example, the tenacity of a yarn relaxed 20% usually runs from 6-8 g./d.

with15-20% elongation. Ifless than,20% relaxationis used, the tenacity is higher and the elongation is lower. The melting point (fora highly crystalline material this,v term .isv substantially synonymous with softening point. or sticking point) of the finished, polymershows no. appreno unnecessary limitations are to be understood therefrom. The invention is not limited to the exact details shown and described for obvious modifications will occur to those skilled in the;

art.

What is claimed is:

1. A process for the preparation of improved polyvinyl alcohol fibers which comprises polymerizing vinyl acetate in a liquid solvent for vinyl acetate monomer and polymer, interrupting the that of the spun filament, Washing and drying. further orienting the dry filaments by stretching the same at a temperature between 200 and 240 C. t a final length at least two and one-half times the length after the first orientation, and relaxing said further oriented fibers by heating the same at a temperature between 210 and 255 C. but not more than 0. below the temperature at which they were further oriented and under tension controlled to prevent shrinkage of more than 35% of said further oriented length and until the filaments shrink less than 10% of their length when immersed for sixminutes. in water at 100 C'.

2. A process for obtaining a water-resistant polyvinyl alcohol filament, said process, comprising polymerizing vinyl acetate in contact with a polymerization catalystand interrupting the polymerization at a polymer concentration in the polymer solution not exceeding 60% by weight of the solution, subjecting the polyvinyl acetate thus obtained to substantially complete hydrolysis, forming an aqueous solution of the resulting polyvinyl alcohol into a filament, drying said filament, and orienting the dried filament by thermally stretching it at a temperature of from 200 C. to 240 C., heating the stretched filament under tension controlled to prevent morethan 35% shrinkage by said heating based on its stretched length, said last-mentioned heating being at a temperature of from 210 C. to' 255 C., but not more than 10 0. below the temperature of the thermal stretching, and continuing said heating until a filament is obtained which shrinks less than 10% of its length when immersed for six minutes in water at 100 0.

3. A process for obtaining a water-resistant polyvinyl alcohol filament, said process comprising polymerizing vinyl acetate in contact with a polymerization catalyst and interrupting the polymerization at a polymer concentration in the polymer solution not exceeding by weight of the solution, subjecting the polyvinyl acetate thus obtained to substantially complete hydrolysis, forming an aqueous solution of the resultin polyvinyl alcohol into a filament, drying said filament, and orienting the dried filament by thermally stretching it at a temperature of from 200 C. to 240 C., heatingthe stretched filament while holding it under controlled tension which permits the filament to shrink from 15% to. 30% of its stretched length, said last-mentioned heating being at a'temperature of from 210 C. to 255 C., but not more than 10 C. below the temperature of thermal stretching, and continuing said heatingluntil a filament is obtained which shrinks less than 10% of its length when immersed for six minutes in water at C.

p 4. A water-resistant polyvinyl alcohol filament oriented by stretching to a total draw ratio of at least 7.5, insoluble in water at 100 C., and which is free from material rendering polyvinyl alcohol insoluble, but which when immersed in water at 100 C. for six minutes shrinks less than 10% of its length.

5. A water-resistant polyvinyl alcohol filament oriented by stretching to a total draw ratio of at least 7.5, insoluble in water at 100 C., and having a saponification numbernot exceeding 10, the saponified acid being acetic acid, said filament consisting only of vinyl alcohol units and up to 1% vinyl acetate units, said filament shrinking less than 10% of its length when immersed for six minutes in water at 100 C.

EDWARD T. CLINE.

PAUL S. PINKNEY. LOUIS PLAMBECK, JR. HALSEY B. STEVENSON.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,233,442 Wiley Mar. 4, 1941 2,236,061 Izard Mar. 25, 1941 2,322,976 Schmitz' June 29, 1943 2,420,565 Rugeley et al. May 13, 194'? 2,447,140 Shelton et a1 Aug. 17, 1948 FOREIGN PATENTS Number Country Date 322,157 Great Britain Nov. 22, 1929 

1. A PROCESS FOR THE PREPARATION OF IMPROVED POLYVINYL ALCOHOL FIBERS WHICH COMPRISES POLYMERIZING VINYL ACETATE IN A LIQUID SOLVENT FOR VINYL ACETATE MONOMER AND POLYMER, INTERRUPTING THE POLYMERIZATION (A) BEFORE THE CONCENTRATION OF THE POLYMER IN THE POLYMERIZATION SOLUTION EXCEEDS 60% OF THE WEIGHT OF THE SOLUTION, (B) BEFORE THE CONVERSION OF MONOMER TO POLYMER EXCEEDS 75%, AND (C) BEFORE THE VISCOSITY OF THE POLYMERIZATION SOLUTION EXCEEDS ABOUT 75 POISES, HYDROLYZING THE POLYMER TO POLYVINYL ALCOHOL OF SAPONIFICATION NUMBER NOT EXCEEDING 10, SPINNING AN AQUEOUS SOLUTION OF THE POLYVINYL ALCOHOL TO FORM A FILAMENT, ORIENTING SAID FILAMENT TO A DRY ORIENTED LENGTH AT LEAST THREE TIMES AS LONG AS THAT OF THE SPUN FILAMENT, WASHING AND DRYING, FURTHER ORIENTING THE DRY FILAMENTS BY STRETCHING THE SAME AT A TEMPERATURE BETWEEN 200* AND 240* C. TO A FINAL LENGTH AT LEAST TWO AND ONE-HALF TIMES THE LENGTH AFTER THE FIRST ORIENTATION, AND RELAXING SAID FURTHER ORIENTED FIBERS BY HEATING THE SAME AT A TEMPERATURE BETWEEN 210* AND 255* C. BUT NOT MORE THAN 10* C. BELOW THE TEMPERATURE AT WHICH THEY WERE FURTHER ORIENTED AND UNDER TENSION CONTROLLED TO PREVENT SHRINKAGE OF MORE THAN 35% OF SAID FURTHER ORIENTED LENGTH AND UNTIL THE FILAMENTS SHRINK LESS THAN 10% OF THEIR LENGTH WHEN IMMERSED FOR SIX MINUTES IN WATER AT 100* C. 